Previous research has indicated that increased activity of the Hypothalamic-Pituitary-Adrenal (HPA) axis enhances the propensity to self-administer drugs of abuse. We have shown that rats with a greater propensity to self-administer drugs of abuse also avoid intake of a natural gustatory reward after it is repeatedly paired with the opportunity to self-administer the drug reward. Under our reward comparison hypothesis, this avoidance behavior is caused by devaluation of the natural reward in anticipation of the availability of the more rewarding drug of abuse. Thus, we hypothesize that individual differences in activity of the HPA axis mediated individual differences in reward comparison behavior. The proposed studies will test this hypothesis by further investigating these differences in reward comparison behavior. The proposed studies will test this hypothesis by further investigating these differences using a behavioral and physiological approach (Specific Aim I) and by employing pharmacological manipulations to assess the role of corticosterone and corticotropin releasing factor in mediating these individual differences (Specific Aim II).